(1) Field of the Invention
The invention concerns manufacturing of a stator hollow structure for a shrouded rotary assembly, and an aircraft equipped with such an assembly.
Therefore, the most general technical domain of the invention is the domain of propulsion rotary assemblies, for creating airflows by rotating blades (i.e. airfoil designs providing a “fan like” effect).
(2) Description of Related Art
Traditional helicopters comprise a main rotor that provides lift and propulsion. Traditional helicopters also comprise an open-type tail rotor. The tail rotor has a transverse axis and is generally acting so to control the yaw movement of the helicopter.
Many types of shrouded rotary assemblies for aircrafts have been proposed. Since the late 1960s, shrouded rotary assemblies are widely used in helicopters. The most renowned shrouded tail rotors for rotary wing aircrafts are called “Fenestron”®.
In short, a shrouded rotary assembly mainly includes a driven rotor system (commonly called rotor) comprising the rotating blades, and a stator hollow structure (commonly called stator). The rotary blades are operated inside an external hollow duct defined by the stator hollow structure.
So, the stator hollow structure comprises an external hollow duct surrounding a central hub casing. The central hub casing is mounted in the external hollow duct via a plurality of profiled guide vanes.
In such assemblies, the profiled guide vanes are rigid beams, fixedly attached between the central hub casing and the stator hollow structure. These profiled guide vanes are shaped with an aerodynamic profile.
The items of the stator hollow structure are manufactured separately as unitary items, so as to be assembled under consideration of tolerances and corrosion prevention. Most of the times, the separated unitary items of such structure include the external hollow duct, the central hub casing and a series of unitary profiled guide vanes, to be assembled by added mechanical means such as rivets.
Therefore special tools for assembling and installation are needed. Besides, operating assembling and installation are time consuming.
In some rotorcrafts, the external hollow duct is a unitary aluminum ducted part. This part is machined to provide a flange allowing adapting of the central hub casing and to have the profiled guide vanes rigidly fixed therein. In such rotorcrafts, the guide vanes are frequently profiled aluminum extrusion parts, which are finished by chemical milling.
Even if some shrouded rotary assemblies comprise composite parts since the early 1980s, e.g. the rotary blades, many recent assemblies still include metallic parts, e.g. attachment means for the central hub casing. Such metallic parts are subjected to corrosion, and may be bulky/heavy/somehow detrimental to aerodynamics, relatively to a so-called integrated structure.
Besides, manufacturing steps like machining, milling and assembling of numerous unitary items can be cumbersome, costly and prone to human errors (proportionally to the extent of non-automated manufacturing steps).
For aerodynamic reasons and to save weight, a slim design of the profiled guide vanes and of the attachment means is preferred.
Such a slim design interferes with capacities of load bearing of these vanes and attachment means. Thus, aerodynamic performance and weight reduction are antagonistic to load bearing. This is especially detrimental in rotary wing aircrafts.
With a slim design, the loads exerted on the guide vanes leads to stress peaks in the tight radii of the attachment means and therefore tends to increase wall thicknesses in the attachment means and in the external hollow duct. Considering the fact that the stator hollow structure is exposed to high fatigue loads due to stressing forces and vibrations, slim designs may be quite sensitive to fatigue cracks.
In view of limitations of slim designs, classical designs may also appear as expensive, vulnerable to corrosion and fatigue cracks, while being too heavy.
The following prior art documents are known: CA2042235, DE102006051867, EP1344634, EP1676775, GB572417, U.S. Pat. Nos. 4,585,391, 5,108,044, 5,251,847, 5,498,129, 5,562,264, 5,605,440, 7,959,105, US2003/235502, US2006/169835, US2007/013242, US2009/152395, US2010/130093, US2011/129341, US2011/217163 and WO2012169906.
The document CA2042235 describes an unmanned flight vehicle wherein two counter-rotating rotors are positioned within a toroid fuselage. The toroid fuselage is a unitary structure from composite materials, having inner integrated struts to support a separated strut on which the counter-rotating rotors are mounted. The toroid fuselage is having separated inner radial wall mounted therein.
The document EP1344634 describes the molding of fixed blades for a helicopter rotor. The fixed blades are attached to a central hub and a peripheral through seal. The fixed blades are having a number of layers of composite material and are polymerized while being compacted between a matrix and a molding surface of a mold.
The document GB572417 describes a classical helicopter, having a tail boom with a counter torque ducted rotor in a fuselage assembly.
The document U.S. Pat. No. 4,585,391 describes a helicopter tail that comprises a part of fuselage and a vertical fin. In the fin, a tunnel is formed downstream of blades of a rotor. The tunnel is provided with a plurality of fixed blades. The fixed blades are adapted to recover the energy of rotation of the air flow passing through the tunnel and are disposed radially with respect to the tunnel.
The plurality of fixed blades are mounted to become integral with an assembly comprising an outer ring, provided with a flange, and an inner ring, the fixed blades being fastened at their ends with the outer ring and inner ring. These two concentric rings comprise notches or interruptions respectively for the passage of arms.
To avoid the outer ring of forming excess thickness with respect to the wall of a divergent portion, this wall comprises the impression of said outer ring. Similarly, the impression of inner ring is provided in a hub. The support arms are distributed equally about a rotation axis, with the fixed blades to form three identical groups separated from one another and each adapted to be inserted in the space between two of said arms.
The document U.S. Pat. No. 5,251,847 describes a light helicopter that comprises a rear anti-torque system. A fuselage comprises a central part extended rearwards by a tail. The anti-torque system is arranged at the end of the tail and comprises an anti-torque tail rotor with rotating blades. A fairing defines an aerodynamic tunnel in a cylindrical shape in which the tail rotor is housed coaxially. The fairing is arranged at the base of a vertical fin. A tail gearbox is coupled to the tail rotor. Fixing arms are provided for fixing the tail gearbox with respect to the structure of the tunnel.
The document U.S. Pat. No. 5,498,129 describes a helicopter anti-torque system arranged with a tail gearbox. The tail gearbox is in a housing cast as a single piece from metal alloy.
The document U.S. Pat. No. 5,605,440 describes composite vanes for a counter torque ducted device. The vanes and a central body are injection molded as a single piece using the RTM method.
The document U.S. Pat. No. 7,959,105 describes an aircraft having a streamlined stator pierced by an air flow duct defined around an axis of symmetry. The aircraft has a shrouded rotor with rotary blades arranged in said static air flow duct. The periphery of the static air flow duct is provided in succession of a first lip, a second lip and of a first rear portion at the side of the duct that is closer to the rear end of the aircraft.
The document US2006/169835 describes the making of a helicopter tail structure with a rotor aperture lined by a single pieced ring with two end collars or flanges. The single pieced ring is made from a composition material with an organic matrix and reinforcing elements, fixed together by solidifying the matrix.
The document WO2012169906 describes a helicopter structure and in particular to composite. The helicopter has a tail boom and a fan duct structure with a fin attached to a section of empennage using a coupling member. The coupling member is an integral combination of a mechanical fastening provided by a head and a chemical bond provided by a cured adhesive. The stator hollow structure is a one-pieced ring made of composite.
The above shows remaining limits and drawbacks pertaining to shrouded rotary assemblies.
Therefore, a wide demand exits for enhanced shrouded propulsion rotary assemblies. At least one of the following improvements would be useful: reduction of weight, of recurring costs and of manufacturing time. Besides, enhancing the aerodynamic and acoustic performances of the profiled guide vanes would be useful in some cases. Increasing the fatigue strength and avoiding corrosion problems would be also highly beneficial in certain conditions.
The invention is therefore useful by offering a stator hollow structure for a shrouded rotary assembly, manufactured from composite angular segments. A predetermined number of such composite segments are defined, so that all the segments are angularly complementary with the stator hollow structure to be obtained when finalized. This means that when all the segments are gathered side-by-side, the resulting juxtaposed arrangement encompass the overall shape of the stator hollow structure.